Thermal batteries generally contain an active metal anode, a cathode and an electrolyte that is solid at normal, ambient temperatures. A current thermal battery, for instance, may comprise a lithium or lithium alloy anode, an FeS.sub.2 cathode and a LiCl/KCl eutectic mixture electrolyte. Because thermal batteries employ an electrolyte that is solid at normal, ambient temperatures, storability of the battery at ambient temperatures is excellent due to the slow kinetics of the solid to solid reaction between the solid electrolyte and the electrodes as well as the inertness of the electrolyte. At normal ambient temperatures, the solid electrolyte provides very high electrical resistivity allowing no current to pass.
When power is needed, thermal batteries are activated by use of a pyrotechnic heat source to rapidly heat and melt the solid electrolyte to a highly conductive liquid. In order to activate these types of batteries, conventional, pyrotechnic heat sources must generally heat the solid electrolyte to over 450.degree. C. in a time period of often less than two (2) seconds. Once activated, if the battery cell can uphold the very high temperature necessary to maintain the electrolyte in its molten state, the batteries could generate power for anywhere from a few seconds up to complete discharge of the battery.
Because of the high temperature of over 450.degree. C. required to melt the solid electrolyte, a large amount of pyrotechnic heating materials must be used. The need for such great quantities of pyrotechnic heating materials adds significantly to the overall weight and size of the thermal battery. This is undesirable. Ideally, one would like to minimize the size and weight of any power source used.
Moreover, once the thermal battery has been activated, it is often a problem maintaining the high temperatures needed to keep the electrolyte in its molten state until power is no longer needed.
In addition, heating the state of the art thermal batteries to such high temperatures in order to melt the electrolytes and activate the battery requires significant time. The time required to activate prior art thermal batteries is on the order of tens to hundreds of milliseconds. It would be desirable to minimize this activation time.
Accordingly, it is desirable to find a suitable electrolyte for use in a thermal battery having a lower melting point than those employed by the prior art thermal batteries; consequently, melting the electrolyte and maintaining it in its molten state would require lesser quantities of heating materials. In addition, it is further desirable to provide a thermal battery cell the activation of which could be accomplished in a lesser amount of time and using lesser amounts of heating materials.
U.S. Pat. No. 4,764,438 (Vaughn et al.) provides a lightweight, thermally activated, solid state, electrochemical power supply which utilizes a solid alkali metal tetrachloroaluminate electrolyte in combination with a transition metal chloride containing cathode. The battery taught is thermally activated at relatively low temperatures of approximately 85.degree. C. to 105.degree. C. The battery taught, however, is not a thermal battery in accordance with the accepted definition of the term "thermal battery." The electrolyte in Vaughn et al. does not melt during the operation of the battery cell. The cell taught by Vaughn et al. is activated below the melting point of the electrolyte.
U.S. Pat. No. 4,117,207 (Nardi et al.) teaches a thermal battery which is activated at a relatively low temperature range of 165.degree. C. to 250.degree. C. Although the temperature taught by Nardi et al. to activate the thermal battery is lower than that needed to activate other conventional thermal batteries, it would be desirable to provide a thermal battery that can be activated at yet even lower temperatures; and hence, require the use of less pyrotechnic heating materials and provide for shorter activation times as well.
Nardi et al. further teaches a conventional method of making a thermal battery, which requires obtaining, and sometimes preparing, each of the cathode, electrolyte and anode materials in pulverized/powder form. Each of these materials is then separately die pressed layer-by-layer in an apparatus, such as the Carver die to provide a thermal battery wherein a solid electrolyte is sandwiched between a cathode and an anode. This method can be time consuming and complex when one considers the various steps that are needed to be employed in preparing the individual component materials and in the pressing of the various component parts to form the final battery product. Vaughn et al. teaches this method as well as a method for producing the batteries therein.
The present invention provides for a novel and relatively easy method for making the thermal batteries within the scope of the invention. This method eliminates the numerous steps needed and employed by the prior art.
There exists a continuing need to develop a thermal battery that is smaller, lighter and more quickly activated than conventional, thermal batteries. Moreover, there is a continuing need to provide a thermal battery that can be activated at temperatures significantly lower than those employed in the prior art thermal battery art. In addition, there is a need to maintain the molten state of an electrolyte once melted after all pyrotechnic heating materials are exhausted. Being able to accomplish this and provide a method for making such thermal batteries in a relatively simple and cost-effective fashion is further desirable. The present invention provides a solution to meet the needs described.